1. TECHNICAL FIELD
My invention relates to grating apparatus designed to provide a relatively lightweight, maneuverable, and ice- and barnacle-repellent means of filtering the intake water needed by electric power generators and other systems demanding a large flow of water from a reservoir.
2. BACKGROUND ART
Power generating systems, be they fossil-fueled, nuclear-based, or hydro-electric, are dependent on an non-interruptable source of water. In general, water intake requirements for public utilities and private factories--especially those within the pulp and paper industry--which incorporate such generators amount to thousands of gallons of water a minute or more. Depending on the application, this demand may alternate with no water flow at all for a period of time. Nearly without exception this water is drawn from some type of open reservoir: lake, river, ocean. In part because of the flow rate of the water drawn in for this purpose, such operations have long been plagued by problems of trash and marine organism entrainment. As initial filtration to eliminate trash, large gratings of one sort or another have been placed at the point where the water first enters the system. The purpose of these initial filters, often referred to as "trash racks," is to allow nearly free flow of water while at the same time intercepting the larger components of water-borne debris. These trash racks have traditionally been made of steel, with typical dimensions of 2-3 feet by 20 feet, and oriented so that their long dimension is nearly vertical. Within the trash rack frame, there are a series of closely spaced longitudinal steel members, typically of dimensions 1/2".times.6".times.20'. In this way the trash rack constitute a highly "porous" interface between the reservoir and the water intake tunnel leading to the generator. It also constitutes a very heavy filter, given its dimensions and the specific gravity of steel: 7.2.
The tops o the nearly-vertical trash racks often extend to the water's surface. This results in the in-flowing water coming from reservoir depths ranging from zero down to 18-20 feet. Under other circumstances the entire length of the racks may be placed far under the reservoir water's surface. This includes circumstances where the racks or their equivalent are placed horizontally on the reservoir bottom.
The trash rack arrangement described above, though dominant within the industry, has been fraught with problems, and for at least 100 years operators and builders of electric power generators--indeed all systems requiring a high flow of water intake--have experimented with variations on the basic trash rack designs as well as with accessories as to be discussed below. These problems--which are inter-related--include: (1) the great weight associated with steel gratings of the size required; (2) the need for periodic painting and other maintenance, including the removal of marine organisms, especially barnacles; (3) the formation of flow-interrupting ice on those gratings located in northern regions.
The problems are inter-related in the sense that the great weight of the steel trash racks becomes particularly onerous the more frequently these racks have to be removed for maintenance or replacement. In order to prevent the steel from oxidizing (corroding) at a high rate (especially in salt water, where they are often placed), a paint coat must be maintained on the trash racks. This in turn is made more difficult because of the necessity to periodically rake the surface of the racks in order to remove debris which has collected and barnacles which have grown and which together impede the flow of water.
In recent years, the environmental protection prohibitions placed on the use of barnacle-repelling paints have worsened this problem, in the sense that it increases the frequency with which steel trash racks need to be rid of barnacles and the like. Because of the gratings' weight, typically equalling several thousand pounds, their removal is not a trivial undertaking. Because of the function they fulfill, they are commonly located so that placement and removal cannot be accomplished directly from land but must be done from a barge anchored just in front of the water intake point, a requirement which introduces many additional complications. In addition to the periodic pulling of the trash racks for maintenance, there is much effort devoted to their cleaning while they are submerged. This in situ cleaning involves various jury-rigged scrappers and brushes being hauled across the trash racks from above, something which tends to damage the racks' protective paint covering, hastening the need for removal and re-painting--especially where sea water immersion is involved.
A more subtle but potentially much more disastrous problem is ice formation on the grating. The consequences of such formation can range from an interruption of power output up to and including the destruction of the generator. (Depending on the particular circumstances, an intermediate level of damage can be the destruction of the trash rack.)
Contrary to common belief, ice can form on or accumulate on objects beneath the water without being linked to surface ice. Although the distinctions are sometimes blurred, the ice forming on submerged structures such as trash racks can be categorized as either anchor ice or frazil ice. The former depends upon a "supercooling" of the water below its normal freezing point in the vicinity of the underwater objects. It also depends for its formation on the presence of nucleation sites at which the initial crystallization of ice from the supercooled water can occur. Frazil ice build-up on the other hand commences with the adherence of already-frozen crystallites which are borne by the water flow. These crystallites may have formed near the surface of an open reservoir (even in the coldest weather it is possible for parts of the reservoir surface to remain open if the water is sufficiently turbulent) and then been pulled down by the flow pattern of the water. Given the right circumstances, frazil ice can arise and completely block off the flow of intake water in a fraction of an hour. Because of its dependence on the entrainment of crystallites, the frazil ice formation is favored by high flow rates, as well as by sub-freezing water temperatures (which enhance the "stickiness" of the crystallites). On the other hand, anchor ice as defined above generally requires still water for its formation, and may only be a problem in those trash racks involved in the on-and-off flow associated with "as needed" operation of the power generator.
The trash rack icing problem has given rise to a diversity of ad hoc solutions. These range from heating the vicinity of the intake so as either to melt the ice crystallites or to heat the racks up to a temperature where the crystallites will not adhere to the trash rack surfaces. This approach is not cost-free even in those installations where there is "waste" heat carried off. Another technique is to cavitate the water in the vicinity of the trash racks, that is, to introduce hypersonic vibrations. The resulting vibrations make it more difficult for ice to form or adhere to the trash rack surfaces. Some operations periodically direct air guns at the racks to dislodge ice; a related approach is to connect a transducer to the metal racks with which to introduce periodic hypersonic vibrations so as to interrupt the sticking of frazil ice and to dislodge that ice which has adhered.
Yet another approach in the battle with icing at the water intake is to locate the water intake at great depths below the surface, at depths below which the frazil ice is thought to form. Although there is some controversy about the existence of such a frazil ice cut-off depth, many workers believe that at a depth greater than 30 feet the water is free of frazil ice crystallites regardless of the air temperature. Thus, by placing the intake filter on the bottom of a sufficiently deep reservoir and orienting it horizontally--as is taught by Jenkner et al. in "Offshore Intake Structure," U.S. Pat. No. 4,594,024--one might escape the icing problem. It has to be noted that just placing the intake system below the putative cut-off depth is not enough, since water is drawn in from a considerable height above the bottom. For example, if one allows a typical height of 15 feet for this in-drawing and accepts the 30-foot figure for frazil cut-off, the intake filter must be located on the bottom of a reservoir in greater than 45 feet of water. (Other workers have reported frazil ice blockage of a horizontal intake at a reservoir depth of 55 feet, though there is no estimate of the thickness of the horizontal layer from which the intake water was being drawn at the time.)
For a recent discussion of the trash rack icing problem and the range of attempted solutions, see "Frazil Ice Problems at Trash Racks," a paper delivered by F.E. Parkinson at the August, 1987, conference: Hydro Operation and Maintenance: Winter Operation--Ice Problems [sponsored jointly by the Canadian Electrical Association, the Electric Power Research Institute, and Hydro Quebec]. This paper also refers to an 1888 engineering publication dealing with the formation of frazil ice at water intakes in northern climes--an indication of the duration of time over which a solution to the problem has been sought.
All of the prior approaches to ridding the intake system of the icing problem introduce significant costs of their own, in terms of out-of-pocket expenses--e.g., for cavitation--or in terms of the severe constraints they impose on the design options available. One often does not have a choice as to what depth and at what orientation one is going to place the water intake point. Thus, it is seen that a simple means limited to the rack itself of reducing the icing problem and one which also makes the racks more durable and more maneuverable constitutes a significant improvement over the prior art. My invention consists of trash racks constructed entirely or nearly entirely of impact- and abrasion-resistant elastomers which are extremely smooth and of great hardness. Because of their effective lack of nucleation sites, these racks will not collect anchor ice and also will greatly retard if not prevent completely the adherence of frazil ice. Because of its strength, the elastomeric material used can be formed into trash racks of the same dimensions (and, consequently, the same porosities) as the traditional steel racks. Furthermore, its density about 1/6 that of steel results in racks which are about 1/6 the weight of the steel ones they replace. It is only recently that the material has been developed which would permit the mold-casting of very hard elastomers into the large elements needed for the racks. The largest elements are the longitudinal members, which can be cast from elastomers, in particular, polyurethane, with a resulting hardness of 75-95 D on the Shore Durometer scale.
The only prior approach of which I am aware that would make use of the ultra-smooth non-sticky surfaces available from certain elastomers involves coating traditional steel racks with thin elastomer films. Experience to date has shown that this material is easily torn, allowing the underlying ferrous metal to corrode and also providing additional ice-formation and barnacle-attachment sites. Attention is called to U.S. Pat. Nos. 4,594,024 and 4,521,306, and to "Frazil Ice Problems at Trash Racks," op.cit., "Solving icing problem at power plant intakes," by Y.G. Mussalli and B. Budziak, pp. 58-59, Power Engineering, August, 1981, and "Frazil Ice Control Using Electromechanical Vibrators and Ice-Resistant Coatings," by Y.G. Mussalli, L.S. Gordon, and S.F. Daly, pp. 1568-1576 in WATERPOWER '87, Brian W. Clowes, editor (1987).